You could pretty easily rotate through proxies just by changing your proxy settings before fetching a page:

my @proxies = (
    'http://proxy.com:1234',
    'http://someotherproxy.com:8080',
    'http://somerandomproxy.com:4123',
);
foreach(@proxies) {
    $ua->proxy('http', $_);
    $ua->get("http://myserver.com/page_of_ad_banners.html");
}
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